Migrate to Docusaurus (#329)

* docs: initial migration to docusaurus

* website: add custom font, update blurbs and icons

* website: update splash

* root: update links to docs

* flows: use .pbflow extension so docusaurus doesn't mangle the files

* e2e: workaround prospector

* Squashed commit of the following:

commit 1248585dca
Author: Jens Langhammer <jens.langhammer@beryju.org>
Date:   Sun Nov 15 20:46:53 2020 +0100

    e2e: attempt to fix prospector error again

commit 1319c480c4
Author: Jens Langhammer <jens.langhammer@beryju.org>
Date:   Sun Nov 15 20:41:35 2020 +0100

    ci: install previous python version for upgrade testing

* web: update accent colours and format

* website: format markdown files

* website: fix colours for text

* website: switch to temporary accent colour to improve readability

* flows: fix path for TestTransferDocs

* flows: fix formatting of tests

website/static/flows/unenrollment.pbflow

@@ -0,0 +1,37 @@
+{
+    "version": 1,
+    "entries": [
+        {
+            "identifiers": {
+                "pk": "59a576ce-2f23-4a63-b63a-d18dc7e550f5",
+                "slug": "default-unenrollment-flow"
+            },
+            "model": "passbook_flows.flow",
+            "attrs": {
+                "name": "Default unenrollment flow",
+                "title": "Delete your account",
+                "designation": "unenrollment"
+            }
+        },
+        {
+            "identifiers": {
+                "pk": "c62ac2a4-2735-4a0f-abd0-8523d68c1209",
+                "name": "default-unenrollment-user-delete"
+            },
+            "model": "passbook_stages_user_delete.userdeletestage",
+            "attrs": {}
+        },
+        {
+            "identifiers": {
+                "pk": "eb9aff2b-b95d-40b3-ad08-233aa77bbcf3",
+                "target": "59a576ce-2f23-4a63-b63a-d18dc7e550f5",
+                "stage": "c62ac2a4-2735-4a0f-abd0-8523d68c1209",
+                "order": 0
+            },
+            "model": "passbook_flows.flowstagebinding",
+            "attrs": {
+                "re_evaluate_policies": false
+            }
+        }
+    ]
+}